The present invention relates to a method of forming structures made of aluminum alloys, particularly of naturally hard AlMg alloys, naturally hard AlMgSc alloys and/or age-hardenable AlMgLi alloys.
In aeronautical and aerospace engineering, complex structures of high strength and stiffness are required which, taking into account their weight as well as aerodynamic aspects, should have an optimal design. Such structures or structural parts include, for example, wing shell surfaces, covering and tank elements for spacecraft, airplane fuselage surfaces with structure reinforcing elements, such as stringers and ribs. As a rule, a manufacturing of such structural parts made of aluminum alloys which has precise contours and corresponds to the drawings is difficult and usually requires several forming steps for the individual components with corresponding intermediate annealing treatments.
The conversion of welded integral constructions in the construction of airplanes requires the use of readily weldable corrosion-resistant materials, such as AlMgSc and AlMgLi alloys. Because of their spectrum of characteristics, these alloys only have a very limited ductility. As a result, a shaping into the desired end contour is partly not possibly by means of conventional methods because the capacity for deformation is insufficient.
It is today's state of the art that the shell areas are formed from metal plates of Alloy AA2024 in the solution-heat-treated condition by means of stretch-forming. It is known that, during stretch-forming, which can be carried out in the cold as well as in the warm condition, the structure to be formed is formed in one or several steps or phases (compare German Patent Document DE 195 04 649 C1). In this case, the structure to be formed can first be stretched in the longitudinal direction and subsequently over a structural part which has the desired end contour.
It is disadvantageous in this case, that as a result of the forming operation, internal tensions are created in the material which, when operating loads are superimposed, may lead to a failure of the structure. Furthermore, a forming into a structure with a spherical curvature, that is, with curvatures along different directions in space, presents difficulties and requires correspondingly designed machines and dimensionally stable tools. In addition, the structure to be formed is usually damaged by the mounting of clamping jaws on the outer edges so that these areas have to be removed, for example, by means of contour milling. This not only results in a loss of material but also requires another machining step which leads to unnecessary expenditures and a connected time consumption.
In addition, in the case of the AlMg alloys, when the forming takes place at room temperature, a discontinuous deformation is observed as well as the forming of characteristic surface phenomena which are also called Luder's lines and may have a disturbing effect on the characteristics of the material.
It was also found that the group of the AlMg alloys have a planar anisotropy with an r-value minimum in the L-direction (rolling direction). This means that the material flow during the stretch forming for the most part takes place from the metal plate thickness and the structure to be formed therefore tends to thin out locally earlier and fail at a premature point in time. In addition, the reduction of the metal plate thickness by stretching has the result that the reaching of a final thickness which corresponds to the drawings can be achieved only by means of uniform degrees of stretching and is therefore difficult to implement in the case of components with large development differences.
It is known that, in addition to stretch forming, an age hardening process is used which is carried out, for example, under the effect of pressure and temperature in an autoclave or furnace and during which an age-hardening effect occurs simultaneously. This so-called “age forming” process is used for age-hardenable Al alloys of the 2xxx, 6xxx, 7xxx and 8xxx series. In this case, an elastic forming of the structure to be formed first takes place under the effect of pressure or force. The structure to be formed conforms to a structural part which has a smaller radius of curvature than the finished component in order to take into account the so-called “spring-back” effect. Therefore, the structure to be formed is first formed beyond its desired final shape. As a result of the subsequent heating to the alloy-specific age-hardening temperature, a deformation takes place with a partial relaxation of tensions, as described, for example, in the article by D. M. Hambrick “Age Forming Technology Expanded in an Autoclave”, SAE Technical Paper Series, General Aviation Aircraft Meeting and Exhibition, Wichita, Kans., Apr. 16–19, 1985, NO. 850885. This has the result that the component springs back to a certain degree during the cooling and will only then assume its final shape. Thus, after the cooling and relieving, the formed structure has a larger radius of curvature than before the heating. This is problematic mainly for the manufacturing of structural parts because the “spring-back” effect has to be predicted with high precision in order to design the structural part in such a manner that the finished component finally assumes the desired final shape. This, in turn, requires a high-expenditure simulation of the “spring-back” effect, as described, for example, in European Patent Documents EP 0517982A1 and EP 0527570B1.
In addition to the age-hardenable alloys used today (for example, AA2024, AA6013, AA6056), new naturally hard, that is, non-age-hardenable alloys have been developed for future airplane generations, which, in contrast to the established alloys, for metallurgical reasons, cannot be solution-heat-treated because this would lead to an irreversible loss of strength. Thus, the new materials cannot be formed without problems by means of conventional methods. As a result, alternatives are required for the production of double-curved or spherical shell areas.
It is therefore an object of the present invention to provide a method by means of which complex structures of the alloys according to the invention can be formed in a simple manner, that is, with as few process steps as possible, without any significant spring-back effect. At the same time, the loss of material as a result of additional machining should be as low as possible.
According to the invention, this object is achieved in that a component which is to be formed and which consists of the alloys according to the invention is elastically formed under the effect of external force and in the process takes up its desired final shape, and in that the elastically formed component is then heated to a temperature which is higher than the temperature required for the creep forming and the relaxation of tensions of the alloy, so that, if possible, the component is formed while retaining its final shape.
In this manner, it is achieved that the component is formed under the effect of heat without any significant spring-back and in the process almost completely retains the final shape impressed by the elastic forming. After the forming and subsequent cooling, the component therefore basically has the same curvature as before the heat treatment. This has the advantage that the structural parts or holding devices used for the elastic forming, with sufficient precision, have the same shape as the theoretical shape of the component and thus a complex simulation for predicting the “spring-back” effect is not required.
The elastic forming of the component before the heat treatment, in which case the component already assumes its desired final shape, can be implemented according to a first embodiment such that, after the component to be formed is inserted into a holding device, an external force acts upon the component, after which the component conforms to the contour of the holding device while being formed elastically. The external force may be transmitted by way of a mechanical pressure or stamping device which presses the component in the direction of the holding device. As an alternative, the elastic forming can take place directly by the effect of an external pressure which is generated, for example, in an evacuated space.
According to another embodiment, it is expedient that an external force act in such a manner upon the component inserted into the holding device that the component bends elastically in the direction of the holding device so that a hollow space is created between the component and the holding device. This hollow space is then sealed off by means of a sealing material and is then evacuated. Because of the resulting vacuum, the component, while being elastically formed, conforms completely to the contour of the holding device and assumes the desired final shape. Subsequently, under the effect of heat, the forming of the component takes place at temperatures which are above the temperature required for the creep forming and the relaxation of tensions of the alloy.
The advantage is therefore not only that the contour of the holding device corresponds to the desired final shape of the component to be formed but also the forming is of a purely elastic nature as a result of the effect of the external forces. This means that the component returns to its original shape when it is no longer affected by external forces. As a result, corrections or another insertion can take place without any problem. The elastic forming of the component by the effect of the external forces can therefore be repeated at any time.
It is also expedient to heat the component at a heating-up rate of from 20° C./s to 10° C./h to a maximal temperature above the temperature required for the creep forming and relaxation of tensions of the alloy and subsequently cool the component at a rate of between 200° C./s to 10° C./h. The maximal temperature is preferably between 200° C. and 450° C. and is typically kept constant for a time period of from 0 to 72 hours.
In this case, it is advantageous that, within the above-mentioned ranges, the heating-up and cooling rate respectively as well as the maximal temperature can be adapted to the used alloy or to the desired physical properties. In addition, after the implementation of the method, another forming of the component can take place which is not possible or is possible only to a limited extent by means of the known methods.
Another advantage of the method according to the invention is the fact that singly curved as well as spherical structures can be formed in one working step. For this purpose, the holding device has curvatures which extend in different directions in space and correspond to the finished final contour of the component to be formed. Furthermore, in addition to 2D structures, complex 3D structures, on which stringers and ribs are already fastened, can be formed in a simple manner. Simultaneously, deformations caused by thermal stress resulting from a preceding welding operation are compensated by the forming process according to the invention.